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The South Platte River alluvial aquifer between Denver and Greeley, Colo., covers an area of about 75 square miles. The predominant land use in the area is irrigated agriculture, which has changed the chemistry and hydrology of the alluvial aquifer in at least two important ways. First, the infiltration of irrigation water has resulted in a buildup of dissolved nitrate in groundwater in the aquifer. Second, it has increased the amount of discharge from the aquifer to the South Platte River. One objective of the study was to determine if naturally occurring processes in the aquifer reduce nitrate concentrations in the water prior to its discharge to the South Platte River, thereby decreasing the effect of irrigated agriculture on water quality in the river.
The U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Pro gram began in 1991 and describes the status of and trends in the quality of the Nation's surface-water and groundwater resources. The program provides a thorough understanding of the natural and human factors that affect the quality of those resources. NAWQA plans to conduct investigations in 60 study areas that represent a variety of geologic, hydrologic, climatic, and geographic condi tions throughout the Nation. The South Platte River Basin study unit, which is in parts of Colo rado, Nebraska, and Wyoming, was among the first 20 study units in which work was begun in 1991.
One objective of the study was to determine if naturally occurring processes in the aquifer reduce nitrate concentrations in the water prior to its discharge to the South Platte River, thereby decreasing the effect of irrigated agriculture on water quality in the river.
The South Platte River alluvial aquifer is the most productive aquifer in the basin; most of its water is used to irrigate overlying cropland. The aquifer consists of unconsolidated clay, silt, sand, and gravel and ranges in thick- ness from about 15 to 60 feet and in width from about 0.5 to 5 miles. Recharge to the aquifer is from infiltration of irrigation water and pre cipitation, and both occur over most of the floodplain and terrace deposits adjacent to the river. Discharge from the aquifer is to the river.
Microbial denitrification is a bacterial process that converts nitrate (NO3) to nitrogen gas (N2). Nitrogen gas is a harmless end product and is the major component in the air we breath.
Long-term agricultural activity on land that overlies the South Platte River alluvial aquifer has resulted in a buildup of dissolved nitrate in water in the recharge area. Nitrate concentrations in groundwater from the recharge area near Greeley ranged from less than 0.1 to greater than 45 milligrams per liter as nitrogen; the median concentration was 26 milligrams per liter as nitrogen. The maximum contaminant level for nitrate is 10 milligrams per liter as nitrogen in drinking water as established by the U.S. Environmental Protection Agency. Measurement of the stable nitrogen-isotope ratios of the nitrate indicated that the nitrate was derived from animal waste-an interpretation that is consistent with the long-term practice in the area of fertilizing fields with ani mal manure from feedlots.
Nitrate concentrations in water from the discharge area near the South Platte River were substantially lower than concentrations in water from the recharge area. Nitrate concentrations in groundwater from the discharge area ranged from about 2 to 30 milligrams per liter as nitrogen; the median concentration was 6 milligrams per liter as nitrogen. The decrease in nitrate concen trations between the recharge and the discharge areas indicated that nitrate concentrations were attenuated along flow paths in the aquifer.
Microbial denitrification in aquifer sediments in the discharge area was at least partly responsible for the decrease in nitrate concentrations between the recharge and discharge areas. For example, concentrations of dissolved oxygen were high in water from the recharge area. Den itrifying activity is inhibited in the presence of oxygen; therefore, nitrate persists in water from the recharge area. In contrast, dissolved oxygen was absent or at very low concentrations in water in the discharge area, which allowed denitrification in sediments in discharge areas. The stable nitrogen isotope ratios of nitrate in the groundwater increased (became more enriched in the heavy isotope 15N compared with the light isotope 14N) as the nitrate was transported from the recharge area to the discharge area. At the same time, the groundwater became enriched in dis solved nitrogen gas, which is the major product of the denitrification reaction. Increases in nitrate nitrogen isotope values and nitrogen gas concentrations along with decreases in nitrate concentra tions further indicated that denitrification in sediments in the discharge area was at least partly responsible for reducing nitrate concentrations in the groundwater before its discharge to the river.
A mass balance based on nitrate concentrations in surface-water inflows and outflows indi cated that the concentration of nitrate in groundwater that discharges to the South Platte River in the study area was much less than what would be predicted on the basis of nitrate concentrations in the water from the recharge area. Differences between groundwater nitrate concentrations in the recharge area and those in the discharge area were observed elsewhere along a 250-mile seg ment of the alluvial aquifer from north of Denver to Julesburg, Colo.; these differences indicate that denitrification may have occurred in discharge areas throughout the alluvial aquifer in Colo rado. Despite the effect of denitrification in reducing nitrate concentrations in groundwater discharge, the process did not completely denitrify the groundwater. As a result, groundwater discharge still affected the quality of water in the South Platte River.
1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal
Center, Lakewood, CO 80225
2 U.S. Geological Survey, Reston, Virginia